Patent Application
20070068649
The invention relates to investment casting. More particularly, the invention relates to investment casting core assemblies.
Investment casting is commonly used in the aerospace industry. Various examples involve the casting of gas turbine engine parts. Exemplary parts include various blades, vanes, seals, and combustor panels. Many such parts are cast with cooling passageways. The passageways may be formed by sacrificial casting cores.
Exemplary cores include ceramic cores, refractory metal cores (RMCs), and combinations thereof. In exemplary combinations, the ceramic cores may form feed passageways whereas the RMCs form cooling passageways extending from the feed passageways through walls of the associated part. The cores may be assembled to each other and secured with a ceramic adhesive. An exemplary ceramic adhesive is alumina-based. For example, the adhesive may comprise alumina powder and a binder such as colloidal silica.
After the initial casting of the part (e.g., from a nickel- or cobalt-based superalloy), the casting shell and core(s) are destructively removed. Exemplary shell removal is principally mechanical. Exemplary core removal is principally chemical. For example, the cores may be removed by chemical leaching. Exemplary leaching involves use of an alkaline solution in an autoclave. Exemplary leaching techniques are disclosed in U.S. Pat. Nos. 4,141,781, 6,241,000, and 6,739,380.
Accordingly, one aspect of the invention involves a method for attaching a metallic casting core to a ceramic casting core. An insertion portion of the metallic casting core is inserted into a receiving portion of the ceramic casting core. A slurry is introduced between the metallic casting core and the ceramic casting core.
In various implementations, the metallic casting core may comprise a refractory metal-based substrate (e.g., optionally coated). The method may be used to form a turbine blade core assembly or a turbine vane core assembly. The slurry may be heated to harden. The metallic casting core and ceramic casting core may be vibrated during the introducing. The inserting may be performed with the ceramic casting core in a green state. The slurry may comprise zircon and aqueous colloidal silica.
Another aspect of the invention involves an apparatus for manufacturing a casting core assembly. The apparatus has means for holding a ceramic casting core. The apparatus has means for holding a metallic casting core with an insertion portion received in a receiving portion of the ceramic casting core. The apparatus has means for vibrating the ceramic casting core and the metallic casting core.
In various implementations, the means for holding may include means for adjusting relative position of the ceramic casting core and metallic casting core.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
In various manufacturing contexts, the binders of ceramic adhesives may have adverse reactions with additional items such as refractory metal cores. As an alternative, a specialized slurry has been developed to secure cores based upon shelling slurries. The exemplary slurry consists essentially of a combination of: zircon and aqueous colloidal silica in a 79:21 weight ratio; a surfactant; and sufficient additional water to achieve the desired viscosity. Exemplary ranges for the zircon to colloidal silica ratio are 70:30 through 80:20.
An exemplary surfactant is essentially a linear alcohol-based surfactant available from Solvay Chemicals, Inc. of Houston, Tex. under the trademark ANTAROX BL 225. An exemplary surfactant amount is 0.05-0.15%, by volume, more narrowly under 0.1% such as 0.7-0.9%. An optional additive is polydimethyl siloxanes (available from Hydrolabs, Inc. of Wayne, N.J. under the trademark BURST RSD-10) in a small amount (e.g., 0.005-0.015%, by volume) to aid in bubble rupture. The exemplary slurry has a density in the range of 2.87-2.96 g/cm3. The exemplary slurry has a pH in the range of 9.0-10.5. The exemplary slurry has a viscosity of 25 (+/−2) centiPoise (cP). However, other viscosities may be appropriate for particular situations, especially thinner slurries (e.g., 18-25 cP).
One or more ceramic cores may also be formed 26 (e.g., of or containing silica in a molding and firing process). One or more of the coated metallic core elements (hereafter refractory metal cores (RMCs)) are assembled 28 to one or more of the ceramic cores. As noted above, the assembly may include use of a ceramic slurry discussed below. The core assembly is then overmolded 30 with an easily sacrificed material such as a natural or synthetic wax (e.g., via placing the assembly in a mold and molding the wax around it). There may be multiple such assemblies involved in a given mold.
The overmolded core assembly (or group of assemblies) forms a casting pattern with an exterior shape largely corresponding to the exterior shape of the part to be cast. The pattern may then be assembled 32 to a shelling fixture (e.g., via wax welding between end plates of the fixture). The pattern may then be shelled 34 (e.g., via one or more stages of slurry dipping, slurry spraying, or the like). After the shell is built up, it may be dried 36. The drying provides the shell with at least sufficient strength or other physical integrity properties to permit subsequent processing. For example, the shell containing the invested core assembly may be disassembled 38 fully or partially from the shelling fixture and then transferred 40 to a dewaxer (e.g., a steam autoclave). In the dewaxer, a steam dewax process 42 removes a major portion of the wax leaving the core assembly secured within the shell. The shell and core assembly will largely form the ultimate mold. However, the dewax process typically leaves a wax or byproduct hydrocarbon residue on the shell interior and core assembly.
After the dewax, the shell is transferred 44 to a furnace (e.g., containing air or other oxidizing atmosphere) in which it is heated 46 to strengthen the shell and remove any remaining wax residue (e.g., by vaporization) and/or converting hydrocarbon residue to carbon. Oxygen in the atmosphere reacts with the carbon to form carbon dioxide. Removal of the carbon is advantageous to reduce or eliminate the formation of detrimental carbides in the metal casting. Removing carbon offers the additional advantage of reducing the potential for clogging the vacuum pumps used in subsequent stages of operation.
The mold may be removed from the atmospheric furnace, allowed to cool, and inspected 48. The mold may be seeded 50 by placing a metallic seed in the mold to establish the ultimate crystal structure of a directionally solidified (DS) casting or a single-crystal (SX) casting. Nevertheless the present teachings may be applied to other DS and SX casting techniques (e.g., wherein the shell geometry defines a grain selector) or to casting of other microstructures. The mold may be transferred 52 to a casting furnace (e.g., placed atop a chill plate in the furnace). The casting furnace may be pumped down to vacuum 54 or charged with a non-oxidizing atmosphere (e.g., inert gas) to prevent oxidation of the casting alloy. The casting furnace is heated 56 to preheat the mold. This preheating serves two purposes: to further harden and strengthen the shell; and to preheat the shell for the introduction of molten alloy to prevent thermal shock and premature solidification of the alloy.
After preheating and while still under vacuum conditions, the molten alloy is poured 58 into the mold and the mold is allowed to cool to solidify 60 the alloy (e.g., after withdrawal from the furnace hot zone). After solidification, the vacuum may be broken 62 and the chilled mold removed 64 from the casting furnace. The shell may be removed in a deshelling process 66 (e.g., mechanical breaking of the shell).
The core assembly is removed in a decoring process 68 to leave a cast article (e.g., a metallic precursor of the ultimate part). Inventive multi-stage decoring processes are described below. The cast article may be machined 70, chemically and/or thermally treated 72 and coated 74 to form the ultimate part. Some or all of any machining or chemical or thermal treatment may be performed before the decoring.
The fixture includes features for holding an associated feedcore 210. These features may include a plurality of tooling balls 222 precisely fixed on the base to engage the feedcore 210. A clamp 224 may be mounted on the base to engage the feedcore after the feedcore is placed against the tooling balls. A pivotal retaining bar 230 may be positioned to engage a root portion of the feedcore to retain the feedcore in position.
The fixture includes features for holding an associated RMC 212 relative to the associated feedcore. In the exemplary engagement, a leading end portion of the RMC is inserted within a slot in a trialing leg of the feedcore. The RMC-holding features may include a clamp 240 grasping a trailing end portion of the RMC. The clamp may be mounted to a gantry structure 242. The exemplary gantry structure is slidably mounted for movement along a direction 500. The gantry (and thus the RMC) position may be controlled by a micrometer mechanism 250. The exemplary micrometer mechanism biases the gantry against the root end of the feedcore to provide fine adjustment of the position of the RMC along the feedcore.
After installation and positioning 300 (
Advantageously, the slurry has a viscosity effective to facilitate its shake-assisted infiltration into the joint. The drying shrinkage, however should not be so great as to risk mechanical failure. Also, the coefficient of thermal expansion should be effective to maintain engagement during the heatings associated with firing and casting. The exemplary properties and composition discussed above are believed particularly effective.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the principles may be implemented as modifications of existing or yet-developed processes in which cases those processes would influence or dictate parameters of the implementation. Accordingly, other embodiments are within the scope of the following claims.
